1. Field of the Invention
The present invention relates to audio speakers. Particularly, the present invention relates to audio speakers that utilize liquid suspension mechanisms for the voice coils incorporated into audio speakers.
2. Description of the Prior Art
Audio speakers produce audible sounds by displacing air by way of the movement of a diaphragm. The diaphragm is attached to and moves under the control of a voice coil, through which electric currents associated with the sounds to be reproduced are driven. The voice coil is disposed in an annular air gap of a magnetic structure. The magnetic structure includes a permanent magnet that provides radial flux in the air gap. Current through the coil interacts with this radial flux to provide axial forces on the coil and causes displacement of the coil and the attached diaphragm.
The alignment of the voice coil in the air gap is crucial to the performance of an audio speaker. Any scraping of the voice coil against the sides of the air gap causes humming and distortion, which affects sound quality. Additionally, scraping creates undesirable stresses on the suspension system as well as removal of the insulation from the coil windings. This leads to early speaker failure. Speaker manufacturers commonly employ a flexible, fibrous element called a spider to align the voice coil in the air gap. The inclusion of a spider requires additional space in the speaker, which is not available in audio speakers such as tweeters and very small full range speakers.
For over 25 years, air gaps in some audio speakers have been filled with magnetic fluids (also called ferrofluids). These ferrofluids offer significant performance advantages in tweeters, midrangers, woofers, compression drivers, and automotive speakers. Ferrofluids are used for a wide variety of reasons such as damping of the voice coil, heat transfer, reduction in harmonic distortion, lubrication, and centering of the voice coil in the air gap.
A unique aspect of ferrofluid is its ability to exert a radial force on the voice coil, which has come to be known as the centering force. The magnitude of this force depends on the magnetization of the ferrofluid and the magnetic flux density in the air gap of the speaker. The higher the magnetization and flux density, the greater the radial force. The radial force keeps the voice coil properly aligned as it vibrates in the gap in response to an audio signal without scraping against the metal pieces, e.g. pole piece and front plate, forming the gap.
In a 1980 AES publication by Bottenberg et al., titled “The Dependence of Loudspeaker Design Parameters on the Properties of Magnetic Fluids,” which was presented at the 61st Convention of the Audio Engineering Society, Nov. 3-6, 1978, the authors derived a mathematical expression for the ferrofluid radial force for a 1 inch dome tweeter having the gap on both sides of the coil filled with a ferrofluid. Many tweeter manufacturers now take advantage of the centering force of ferrofluid by filling the gap on both sides of the coil and thus requiring no mechanical centering device, i.e. a spider.
Unfortunately, the use of ferrofluid in the air gap causes other problems. The air gap is connected to a cavity between the speaker magnet and various components of the magnetic structure. Ferrofluid in the air gap acts like an O-ring seal between the voice coil and the magnetic structure. The cavity is essentially sealed off from the air space in front of the front plate. As the voice coil moves or as the temperature rises, it tends to elevate the air pressure within this cavity. If the pressure builds up to a point where it exceeds the pressure capacity of the ferrofluid O-ring seal, the air bursts through the seal and relatively large amounts of the ferrofluid may then be blown or flow out of the gap. One solution to this problem was addressed by U.S. Pat. No. 5,335,287.
U.S. Pat. No. 5,335,287 (1994, Athanas) discloses a loudspeaker with a viscous magnetic fluid suspension for the voice coil rather than the corrugated disk suspension that is conventionally used. Specially designed vent passages are formed in the magnet assembly in order to prevent internal pressure from either building up or creating sub-atmospheric conditions that could cause the magnetic fluid to be blown out of the magnetic gap. The patent also discloses that the mechanical centering device can be eliminated even in more demanding high excursion woofers by utilizing ferrofluids of high magnetization values such as 600-800 Gauss filling the gap on both sides of the voice coil.
Other problems that arise when using ferrofluid in the air gap around the voice coil involve equalization of the amount of ferrofluid on both sides of the voice coil (because filing the entire gap can be problematic) and equalization of the air pressure under the dust cap of the voice coil. To address these problems, a plurality of vent openings were incorporated in the voice coil adjacent the dome end of the voice coil such as is disclosed in U.S. Pat. No. 4,414,437.
U.S. Pat. No. 4,414,437 (1983, Trauernicht et al.) discloses a moving coil dynamic transducer. The electromagnetic transducer (voice coil) includes a member producing a magnetic field and presenting an air gap traversed by the magnetic field, a mass of magnetic fluid extending across the air gap, and a moving coil mounted on a moving coil carrier supported for movement through the air gap. The moving coil carrier is provided with at least one passage located to communicate with the magnetic liquid during at least part of the movement of the coil carrier through the air gap. The passage permits flow of magnetic liquid from one side to the other of the carrier in the direction of the air gap. This provides an equalization of the magnetic liquid on each side of the voice coil.
Therefore, what is needed is an audio speaker that uses ferrofluid for centering a voice coil without the need for ferrofluid equalizing vent openings. What is also needed is an audio speaker that uses ferrofluid for centering a voice coil without the need for ferrofluid equalizing vent openings or the use of a spider.